1. Field of the Invention
This invention relates generally to a novel process for the production of N-(hydroxyphenyl) maleimides and, more particularly, to a process for producing such maleimides by dehydrative cyclization of N-(hydroxyphenyl) maleamic acids, obtained by the reaction of aminophenols with maleic anhydride, with the use of a dehydrating agent selected from oxides and oxyacids of phosphorus or sulfur, and alkali metal and alkaline earth metal salts of the oxyacids thereof, or by hydrolysis of esters of N-(hydroxyphenyl) maleimides in the presence of at least one of said dehydrating agents. The ester may be obtained by the reaction of aminophenols with maleic anhydride in the presence of a conventional imideforming catalyst and an acid anhydride serving as a dehydrating agent.
2. Description of the Prior Art
For the production of maleimides, a process is generally known in which an amine is reacted with maleic anhydride in the presence of a catalyst such as triethylamine and sodium acetate using acetic anhydride as a dehydrating agent. Such a process, however, is not applicable when the amine is an aminophenol having one or more hydroxyl groups on its phenyl nucleus because the reaction inevitably involves esterification of the hydroxyl group due to its high reactivity, which takes place prior to the cyclic condensation reaction, resulting in the failure to yield a desired N-(hydroxyphenyl) maleimide. Further, even with the use of a catalyst generally known to be effective, such as a metal salt including sodium acetate, cobalt acetate, nickel acetate, manganese acetate, hydrates thereof and the like metal salt, and a base including triethylamine, N-dimethylbenzylamine, isoquinoline, triethylenediamine, pyridine, N-ethylmorpholine and the like base, in conjunction with an effective dehydrating agent such as an acid anhydride including propionic anhydride, butyric anhydride, benzoic anhydride and the like, the hydroxyl group is first esterified with or without subsequent cyclic condensation so that the use of such catalysts and dehydrating agents is not effective for the production of N-(hydroxyphenyl) maleimides.
To produce N-(hydroxyphenyl) maleimide derivatives, therefore, a method has been provided wherein the maleimide obtained in the abovementioned manner and, thus, containing an ester group is subjected to a transesterification or hydrolysis treatment, such as disclosed in Belgian Pat. No. 613,801 in which N-(p-hydroxyphenyl) maleimide is produced by transesterification of N-(p-acetoxyphenyl) maleimide.
However, the transesterification requires considerable time, e.g. over 14 hours. In addition, it has been revealed from extensive examination that the yield of N-(p-hydroxyphenyl)-maleimide by transesterification with methanol for 21 hours is as low as 38%, and that the reaction requires a large amount of methanol. Therefore, the conventional method is not economical on an industrial scale.